In conventional regenerator type ultra-low temperature refrigerators, the regenerator at the final cooling stage (the lowest temperature stage) is filled with a metal based magnetic regenerator material such as Er3Ni or HoCu2 or the like, enabling cooling to be performed at 10 K or lower temperatures (Japanese Patent Laid-open Publication No. Hei 5-71816).
However, as shown by the example of HoCu2 in FIG. 1, because these metal based magnetic regenerator materials do not have an adequately large specific heat in the vicinity from 4.2 K to 7 K, their refrigeration performance in the vicinity of 4.2 K is unsatisfactory. Furthermore, these metal based magnetic regenerator materials also suffer other problems in that the associated production costs are high, meaning they are not cheap.